This invention relates generally to residential furnaces, and more particularly, to the control of energy use in a furnace having both an electronic ignitor and a blower which is adapted for continuous use.
Residential furnaces operate in response to thermostat settings to cyclically operate through the heating cycle each time the sensed temperature in the space reaches a predetermined lower level and to shut off when the sensed temperature reaches a predetermined higher level. Initiation of the combustion process at the start of each heating cycle has traditionally been accomplished by way of a constantly burning pilot light. However, in the interest of economy, it has become common practice to replace the pilot light with an electronic ignition system commonly referred to as a hot surface ignitor. Such a device is prompted by a control system to turn on when ignition is desired and to turn off when a flame has been detected. It is therefore off most of the time but, since it operates on the basis of electrical resistance, it draws a substantial amount of current (i.e. 4 to 5 amps) when it is on.
In a conventional heating cycle of a residential gas furnace, when a call for heat is made by the thermostat, the inducer is first turned on by the control to purge the system of unwanted gases. The ignitor is then turned on and allowed to heat up for a short period of time and then the fuel supply is turned on. When ignition occurs and is then detected by a flame sensor, the ignitor is turned off and, after a s.ituable delay for heating up the heat exchangers, the circulating air blower is turned on to move the heated air out to the duct to be circulated throughout the house. It will thus be recognized that, during this normal mode of operation, the blower is always off when the ignitor is turned on and does not come on until the ignitor is turned off.
It has now become desirable at times to operate the circulating blower on a continuous basis, irrespective of whether the burner is turned on or not. One reason for this trend is that, because of fuel economy, houses are built much tighter and the natural air circulation is therefore reduced. Another reason is to make better use of the electrostatic air cleaners which are becoming common in use to improve indoor air quality.
The motor that is used to drive the circulating air blower in a forced air furnace is of a substantial size (e.g 115 volt, 3/4 Horsepower) and can therefore draw a substantial amount of current during operation. For example, in a furnace which does not have an air conditioning system incorporated therein, and therefore does not have a coil to reduce the air flow rate, the circulating air blower motor may draw as much as 12 amps during steady state continuous operation. Thus, if the ignitor is turned on for a heating cycle when the circulating air blower is operating on a continuous basis, the total current draw to the furnace will be greater than 15 amps and will therefore cause a 15 amp fuse to burn out or a circuit breaker to trip. This, in turn, will require at least a change in the fuse, which is inconvenient, and may be cause for other inconvenience and expense to an operator that may not be aware of the cause of the problem.
It is therefore an object of the present invention to provide an improved control system for a residential furnace having an electronic ignition system.
Yet another object of the present invention is the provision in a forced air furnace for continuous operation of the blower without inconvenience to the operator.
Still another object of the present invention is the provision in a residential furnace having an electronic ignitor, for the use of the circulating blower in a continuous mode of operation.
Yet another object of the present invention is the provision for a residential furnace ignition control system which is economical to manufacture and effective in use.
These objects and other features and advantages become readily apparent upon reference to the following description when taken in in conjunction with the appended drawings.